This section provides background information related to the present disclosure which is not necessarily prior art.
Lithium-ion (Li-ion) battery packs for plug-in hybrid electrical vehicle (PHEV) applications are often assembled with high-capacity, high-power pouch cells. The PHEV battery packs have two operation modes: a charge-depleting or electrical-vehicle (EV) mode and a charge-sustaining or HEV mode. The pack capacity and power are determined by the range and maximum power required by the vehicle in the EV mode.
During the EV mode operations, the state of charge (SOC) of the battery cells typically drops from 90 percent to 25 percent (i.e., the usable capacity ΔSOC=65 percent). The EV-mode electrical energy consumption falls in a range of 300˜400 Wh/mile for most passenger cars and light-duty (LD) trucks. For a PHEV with 400-Wh/mile energy consumption and a 40-mile EV range, the required nominal capacity for the battery pack would be 25 kWh.
PHEV battery packs operate at their full capacities only in the EV mode because the vehicle is powered solely by the battery pack in this mode. Because the pack duties are much greater in the EV-mode than those in the HEV mode (the pack functions only for power assistance as in a full-hybrid vehicle), the heaviest thermal load for a PHEV pack is encountered at the end of the EV mode.
The maximum cell temperature (ΔTcell,max) and the maximum differential cell temperature (ΔTcell,max) are important factors to the cell durability. For high-capacity and high-power Li-ion pouch cells, the criteria for the battery cooling system design are often set as Tcell,max≤55° C. and ΔTcell,max<8° C. In order to minimize the number of battery cells in the pack, the battery cells in PHEV packs are generally much larger than those in HEV packs in both capacity and size.
The highest or maximum cell temperature is usually located near the terminal tab region where the highest local current densities are encountered. During continuous discharge with high cell currents in the EV mode operations, the ohmic heat generated in the tabs/busbars can have significant influence on the local cell temperatures near the tabs. Hence, cooling of a PHEV pack often involves not only dissipating the heat generated in the cells but also the ohmic heat generated at the tabs/busbars, which presents a challenge in designing thermal management systems for cooling large-pouch cells in PHEV battery packs.